Pure metals and their alloys tend to combine with the elements of a corrosive medium to form stable compounds. These compounds are referred to as the corrosion product and the metal surface is said to be corroded.
While corrosion can take several forms, it is usually confined to the metal surface. Additionally, corrosion can also occur along grain boundaries because of differences in chemical resistance or local electrolytic action.
In most aqueous systems, the corrosion reaction has anodic and cathodic components which occur simultaneously at discrete points on the surface of the metal. Electrons flow from an anodic site on the metal surface to a cathodic site on the metal surface to equalize the potential.
In many applications, it may not be economical to use the most corrosion resistant material or a material with satisfactory corrosion resistance may not be known. Other methods of avoiding corrosion, such as the use of glass, ceramic, and organic coatings, may also be prohibitively expensive or incompatible with other process conditions. Similarly, the use of sacrificial anodes to achieve cathodic protection may not achieve satisfactory results or be otherwise practical in many applications.
An alternative approach is to minimize the corrosion by adding inhibitors to the corrosive medium. The use of corrosion inhibitors is generally most attractive in closed or recirculating systems in which the annual cost of inhibitor is low. However, inhibitors have also proven to be economical in many once-through systems, such as those encountered in petroleum-processing operations. Inhibitors are effective due to their controlling influence over the reactions which occur at the cathode or anode sites on the metal surface.
Examples of inhibitors used for minimizing corrosion of iron and steel in contact with aqueous solutions are the soluble chromates and silicates. Some organic sulfide and amine materials are also effective in minimizing corrosion of iron and steel in acid solutions. Such inhibitors may be effective for use with stainless steel and other alloy steel materials as well.
One disadvantage of many known inhibitors is that these inhibitors are only soluble in the aqueous phase. However, metals are subject to corrosion in oil and gas pipelines, transmission lines, and wells, as well as aqueous systems such as water-disposal systems and processing equipment such as separators, water treatment units, and dehydration units. This type of equipment is subject to damaging corrosion which can require expensive replacement or cause catastrophic failure. An extensive amount of time and money has been spent attempting to minimize or eliminate corrosion in a wide variety of materials and environments.
Therefore, there is a need for a corrosion inhibitor system that is considerably more effective at protecting ferrous and nonferrous metals against attack by a corrosive environment than corrosion inhibitors available in the past. There is also a need for methods of preventing metal corrosion in petroleum and chemical processing equipment, including pipelines, wells, and water treatment units.